1. Field of the Invention
This invention relates generally to the field of charged particle optics, and more particularly to electron detector optics for high throughput large substrate electron-beam testing systems.
2. Description of the Related Art
The use of electron beams to inspect and electrically test flat panel display substrates is an established technique. The different testing strategies may be characterized by the method of obtaining the test signal from each pixel in the display: mechanical probe testing; electron-beam probe testing; and voltage imaging.
Mechanical probe testing of a flat panel display substrate is illustrated in FIG. 13. During manufacture, all the signal lines 1310 on the display substrate are connected together to one or more signal line shorting bars 1306. Similarly, all the gate lines 1312 are connected together to one or more gate line shorting bars 1308. To connect the mechanical probe testing system to shorting bar 1306, a mechanical probe 1305 physically contacts the signal line shorting bar 1306 and also is connected to the system ground, as shown. A second mechanical probe 1307 connects to the gate line shorting bar 1308 and is also connected to voltage supply 1314. Pixel electrode 1303 is connected to the control transistor 1311 by line 1309 and to an electrometer 1304 by means of support arm 1301 and a mechanical probe 1302. The voltage on the pixel electrode 1303 is measured by the electrometer 1304. The physical probe 1302 supplies a testing current to the pixel electrode 1303 and hence to the control transistor 1311. Testing of the pixel electrode 1303 and control transistor 1311 is then performed by monitoring the electrical response to the charging current using the electrometer 1304. Capacitor 1313 is formed by the overlap between the gate electrode of the control transistor 1311 and the pixel electrode 1303. Typical measurements made include the following: absence/presence of shorting between neighboring pixel electrodes, breaks or shorts in the connections to the control transistor 1311, excessive leakage currents due to too-low isolation resistance in the pixel electrode 1303. The signature of a properly-functioning pixel drive circuit is characterized, as well as the signatures resulting from various pixel malfunctions, such as shorted or open lines, degraded insulating regions within the pixel element, neighboring pixel elements shorted together, etc. Thus mechanical probe testing allows identification of various pixel defects.
Electron-beam probe testing is illustrated in FIG. 14 and is similar to mechanical probe testing, described above, except the third mechanical probe 1302 (shown in FIG. 13) has been replaced by an electron beam 1420 which supplies the charging current to the pixel element 1403. 1405-1414 are equivalent to 1305-1314, described above with reference to FIG. 13. The impact of electron beam 1420 with pixel element 1403 causes the emission of secondary electrons 1421 from pixel element 1403. Secondary electrons 1421 are collected by detector 1422 to form a voltage contrast signal (similar to the signal generated by electrometer 1304 in FIG. 13). The many advantages of electron-beam probe testing over mechanical probe testing are: no-contact and thus no risk of contact damage; faster selection between pixel elements to test; and the opportunity for fast rechecking of all pixels failing a first-pass testing procedure. The electron beam 1420 is generated by an electron optical column; there are some examples in the prior art of testing systems with multiple columns (typically 2-4), each column producing a single electron beam.
Voltage imaging is illustrated in FIG. 15, where all the pixels in display substrate 1550 are being inspected in parallel. Light 1556 from light source 1555 illuminates a splitter mirror 1553. Reflected light 1557 off the upper surface of the splitter mirror 1553 illuminates the under surface of an electro-optic modulator 1552. An optional interface card 1551 may be interposed between the display substrate 1550 and the upper surface of the electro-optic modulator 1552 to improve the coupling of the voltages on the display substrate 1550 to the electro-optic modulator 1552. Due to the electro-optic interaction of the voltages on the display substrate 1550 with the modulator 1552, the reflectivity of light 1557 off the lower surface of the modulator 1552 is affected. Light 1558 represents that fraction of light 1557 which is not reflected off the splitter 1553, instead passing downwards through the beam splitter 1553 to be collected by a CCD camera 1554, which is coupled to display electronics (not shown) to generate a voltage image of the display substrate 1550. A significant disadvantage of this method is the need to fabricate a new modulator 1552 for each new display design.
All three of the flat panel display substrate testing systems and methods described above suffer from throughput limitations which will only get worse as the size of display substrates continues to increase. There is a need for flat panel display substrate testing systems and methods that have higher throughput and that are more readily scalable to larger substrates.
For flat panel display testing, it is also desired to test 100% of the pixels on the substrate surface since typically a display with more than a few defective pixels is unusable. In some cases, if defective pixels are detected early enough in the manufacturing process, these pixels can be repaired. In other cases, if a substrate is found to have numerous defective pixels, it is more economical to scrap that substrate prior to further processing. Substrate testing also provides process feedback: if successive substrates show increasing numbers of defective pixels, a deviation from proper process parameters (etch, deposition, lithography, etc.) may have occurred, which must be corrected quickly to restore normal production yields. There is a need for inspection systems that are able to test 100% of the pixels on a FPD substrate with a high throughput.
Electron beam systems employed for testing or inspection purposes typically generate one or more primary electron beams (or “probes”) which are focused onto the surface of a substrate by probe-forming optics. When the primary electron beam strikes the substrate surface, it generates both secondary electrons and backscattered electrons (SE/BSEs) as is familiar to those skilled in the art. The SE/BSEs are emitted over a wide angular range, spreading out over a distance which may exceed several mm in extent at the detector collection surface. The signal detection process generally involves the collection of secondary electrons (SEs) and/or backscattered electrons (BSEs). Typically, there will be at most one SE detector and/or one BSE detector per primary electron beam. When the test system comprises multiple electron beam columns, with multiple SE and BSE detectors, efficient collection of SEs and BSEs may be compromised by cross-talk between neighboring columns. Cross-talk occurs when the SEs and/or BSEs from one column are collected by the detectors associated with a neighboring column. There is a need for multiple column electron beam test systems for FPDs that are designed to minimize cross-talk between neighboring columns' detectors.
There is a need for multiple electron beam test systems which meet the three requirements of: high throughput; with 100% pixel testing; while avoiding intercolumn crosstalk between testing signals.